Heretofore, a rotary drive apparatus such as a motor or the like for converting the pressure of a fluid into rotary motion has widely been used. Known motors of this type include a vane motor, a gear motor, and a piston motor, etc. (see, for example, Japanese Laid-Open Patent Publication No. 09-303273, Japanese Laid-Open Patent Publication No. 2010-265797, and Japanese Laid-Open Patent Publication No. 60-204978).